This invention relates to aircraft gas turbines and, more particularly, to the stationary shroud used in such gas turbines.
In an aircraft gas turbine (jet) engine, air is drawn into the front of the engine, compressed by a shaft-mounted compressor, and mixed with fuel. The mixture is burned, and the hot exhaust gases are passed through a turbine mounted on the same shaft. The flow of combustion gas turns the turbine by impingement against an airfoil section of the turbine blades and vanes, which turns the shaft and provides power to the compressor. The hot exhaust gases flow from the back of the engine, driving it and the aircraft forwardly.
The turbine blades are mounted on a turbine disk, which rotates on a shaft inside a tunnel defined by a cylindrical structure termed the stationary shroud. (This xe2x80x9cstationary shroudxe2x80x9d is distinct from the rotating shroud found on some turbine blades.) The hot combustion gases flow from the engine""s combustor and into the tunnel. The hot combustion gases pass through the turbine blade structure and cause it to turn. To achieve a high efficiency, it is important that as little as possible of the hot combustion gases leak around the turbine by passing through the clearance between the outermost tip of the turbine blade and the innermost surface of the stationary shroud. However, the sealing of the turbine structure against such leakage presents a problem, because the components of the structure expand and contract differently during the temperature changes of over 2000xc2x0 F. that are experienced during each cycle of engine operation.
To prevent the leakage of hot combustion gases around the turbine, it is known to size the components so that there is initially a very small gap between the blade tips of the turbine blades and the inner surface of the stationary shroud. As the turbine blades heat and expand when the engine is first operated, there is a contacting between the blade tips and the stationary shroud as the turbine turns. Further contacting between the blade tips and the stationary shroud sometimes occurs again during operation of the engine in the usual operating conditions or under unusual conditions such as application of emergency power or large loads applied to the components.
In these circumstances, material is worn away from the blade tips and/or the stationary shroud, and the clearance gap between the blade tips and the inner surface of the stationary shroud increases. Over time the efficiency of the gas turbine declines because increasing amounts of hot combustion gas leak through the enlarged clearance gap. When the efficiency has decreased by an amount that justifies a repair, the blade tips are repaired by welding new material onto the blade tips to lengthen them back to about their original length, or the stationary shroud is replaced, or both.
The repairs and/or replacements are operable, but they may be costly and there is a decrease in gas turbine efficiency until such time as the repairs and/or replacements are made. There is therefore a need for an improved approach to retaining turbine efficiency in gas turbine engines.
The present invention provides a stationary shroud used in a gas turbine engine. The stationary shroud achieves the same sealing function as a conventional stationary shroud, but it is more resistant to heat-induced damage. The present approach is highly versatile. The abrasion or abradability properties of the stationary shroud may be established by control of its composition and structure. That is, it may be made abradable so that the stationary shroud is worn away and the turbine blade tip is affected in a lesser way, or it may be made highly abrasive so that the turbine blade tip is worn away and the stationary shroud is affected in a lesser way. The stationary shroud is compatible with conventional attachment procedures.
A gas turbine stationary shroud comprises a curved stationary shroud body having a concave inner gas-path surface and a generally convex back. The curved stationary shroud body comprises an open-cell solid ceramic foam made of ceramic cell walls having intracellular volume therebetween.
The properties of the ceramic foam may be varied widely according to the requirements of the gas-path surface and the back. At least a portion of the intracellular volume is filled with a metal, such as a nickel-base alloy. Another portion of the intracellular volume may be filled with a material to modify the abradability of the ceramic foam material. Other portions of the intracellular volume may be empty space, or xe2x80x9cporousxe2x80x9d. In a typical case, the ceramic comprises a base ceramic material consisting of aluminum oxide. The stationary shroud desirably comprises at least about 60 volume percent, more preferably from about 60 to about 80 volume percent, of the ceramic. To increase the abrasiveness of the gas-path surface, the ceramic may have an abrasive ceramic mixed with the base ceramic, where the abrasive ceramic is more abrasive than the base ceramic. To decrease the abrasiveness of the gas-path surface, or alternatively stated to increase its abradability, an abradable ceramic may be mixed with the base ceramic, where the abradable ceramic is more abradable than the base ceramic.
In one embodiment, the stationary shroud comprises a support region adjacent to a portion of the generally convex back, with the intracellular volume of the support region being filled with a nickel-base alloy. There is a gas-path region adjacent to the gas-path surface, with the intracellular volume of the gas-path region being porosity. A source of a cooling gas may communicate with the gas-path region at a location remote from the gas-path surface, so that cooling gas flows outwardly through the porosity in the gas-path region.
In another embodiment, the intracellular volume of the support region is filled with a nickel-base alloy, and the intracellular volume of the gas-path region is filled with a metal whose abrasiveness is greater than that of the nickel-base alloy.
In yet another embodiment, the ceramic cell walls of the gas-path region occupy a gas-path-region ceramic volume fraction, and the ceramic cell walls of the support region occupying a support-region ceramic volume fraction that is less than the gas-path-region ceramic volume fraction.
The present approach is compatible with conventional attachment procedures used for stationary shrouds. An attachment integral with the support region is usually provided for this purpose. The embodiment having a reduced support-region ceramic volume fraction is preferred for this application to reduce the abrasiveness of the stationary shroud so that it does not wear away the stationary shroud support.
The stationary shroud is preferably prepared by providing a piece of a sacrificial ceramic having the shape of the shroud, and contacting the piece of the sacrificial ceramic with a reactive metal which reacts with the sacrificial ceramic to form the ceramic foam. The resulting ceramic foam may be further processed to the various embodiments, typically by removing some of the ceramic cell walls and/or removing/replacing the intracellular metal in the required locations.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.